PRELIMINARY TAPHONOMIC ANALYSIS OF A MIDDLE TRIASSIC (PEROVKAN) AMPHIBIAN BONEBED IN NORTHEASTERN NEW MEXICO, USA

Discovered in the 1990’s by a Museum of Northern Arizona field party, the Anton Chico amphibian bonebed lies in the uppermost meter of the Perovkan (early Anisian) Anton Chico Member of the Moenkopi Formation, directly beneath its unconformable contact with the Tecolotito Member of the Upper Triassic Santa Rosa Formation of the Chinle Group. The 20 cm- to 30 cm-thick, greenish gray to brick red, sandy to silty bonebed produces a monodominant assemblage of the Capitosaurid temnospondyl amphibian, Eocyclotosaurus together with a few archosaur teeth and sparse possible reptile bones. To date, the size range of the amphibians collected by New Mexico Museum of Natural History is represented by skulls with a midline length of 180 mm to 380 mm. The extent of the bonebed is unknown, but it contains an MNI of at least many tens of individuals. Based on observed bone density and lateral continuity, hundreds could be present. Preservational quality varies greatly throughout the quarry. Complete disarticulation appears to be the norm, but some possible articulated or associated material is suspected. Conchostracans and small bivalves are found a few cm above the bones, and a 10 cm-thick sandstone beneath them produces sparse fish teeth. The bonebed comprises three layers. In the upper and lower layers dense bones generally lie flat on the bedding planes whereas the intermediate layer has fewer bones that often show significant dip.

The conchostracans and bivalves suggest a pond setting while the monodominant character of the bonebed indicates a probable biological aggregation of the amphibians in that setting. A wide range of size classes (ages) were present; currently, the length ratio of largest/smallest = 2.1, which corresponds to a factor of 9.4 in body mass. Very small juveniles have not yet been found. The presence of some pristine skulls with complete, needle-sharp dentition suggests little or no transport. In other skulls and jaws, the bone appears more weathered, and all the teeth are missing. This highly variable state of preservation probably indicates an attritional assemblage. The interesting three-layered bonebed where more sparse bones with high dip angles lie between dense flat-lying layers may indicate an intermediate period of turbulent influx such as a storm surge that reworked bones from the lower portion of the bed.